This invention relates to a process for the surface treatment of articles fabricated of austenitic iron-nickel-chromium alloys, to resist and deter the onset of intergranular cracking and corrosion and to enhance the concentration of special grain boundaries. The process comprises at least one cycle of working to induce deformation of the near surface region, for example by high density shot peening, followed by recrystallization heat treatment. The novel process can be applied to wrought, cast or welded materials, and is particularly suited for in-situ or field application to components such as steam generator tubes, core reactor head penetrations of nuclear power plants, recovery boiler panels used in the pulp and paper industry, closure welds on canisters for the storage of nuclear waste and storage battery components.
The prior art primarily describes the use of surface cold work, for example by xe2x80x9cshot peeningxe2x80x9d, as a means to effect a state of residual compression at the surface of a material, and thus render the material resistant to the initiation of cracks which require a tensile stress for initiation and propagation. Shot peening is a method of cold working, inducing compressive stresses on and near the surface layer of metallic parts. The process consists of impinging the test article with a stream of shot, directed at the metal surface at high velocity under controlled conditions.
Although peening cleans the surface the major purpose is to impact and enhance fatigue strength. The peening process is known to relieve tensile stresses that contribute to stress-corrosion cracking. Yamada in U.S. Pat. No. 5,816,088 (1998) describes a surface treatment method for a steel work piece using high speed shot peening. Mannava in U.S. Pat. No. 5,932,120 (1999) describes a laser shock peening apparatus using a low energy laser. Harman and Lambert in U.S. Pat. No. 4,481,802 (1984) describe a method of peening the inside of a small diameter tube in order to relieve residual tensile stresses.
Friske and Page in U.S. Pat. No. 3,844,846 (1974) describe a surface deformation treatment by shot peening, which is applied to austenitic Crxe2x80x94Fexe2x80x94Ni alloys without subsequent heat treatment, in order to render the surface region highly deformed, and subsequently more resistant to intergranular corrosion in the event that the article becomes exposed to sensitization temperatures, i.e., 400xc2x0-700xc2x0 C., during service.
Kinoshita and Masamune in U.S. Pat. No. 4,086,104 (1978) also describe a surface deformation treatment for austenitic stainless steel components, applied following final mill annealing or hot rolling treatments, which renders the surface of the stainless steel more resistant to oxide scale formation during subsequent exposure to high temperature steam.
Anello in U.S. Pat. No. 4,495,002 (1985) describes a three step process for martensitic stainless steels to increase their resistance to chloride corrosion, wherein, an article is subjected to surface deformation via shot peening, followed by an ageing treatment at 527xc2x0 C.-549xc2x0 C., and followed by a final lower intensity shot peening. In such manner, a homogeneous near surface region consisting of aged martensite is obtained which is resistant to chloride corrosion and cracking.
Polizotti in U.S. Pat. No. 4,424,083 (1984) discloses a method for enhancing the protection of cast austenitic stainless steel tube against carburization when such tubes are employed in high temperatures carburizing atmospheres, such as in the steam cracking of hydrocarbons. The diffusion of carbon into the alloy steel causing formation of additional carbides, resulting in embrittlement of the tubes, is avoided by heating the cold-worked inner surfaces of such a tube for an effective amount of time, at a temperature between the recrystallization temperature and its melting temperature, in an atmosphere where the oxygen partial pressure is at least oxidizing with respect to chromium. These temperatures used by Polizotti are stated to be 420xc2x0-1150xc2x0 C., preferably 420xc2x0-800xc2x0 C. with the treatment time at such temperatures being about 200 to about 500 hours. Suitable atmospheres include hydrogen or steam. The treatment time required depends on the oxygen partial pressure, longer treatment times are required if the oxygen partial pressure is low.
Palumbo in U.S. Pat. Nos. 5,702,543 (1997) and 5,817,193 (1998), describes thermomechanical mill processes involving the application of bulk cold work followed by recrystallization heat treatment to improve the grain boundary microstructure of austenitic Nixe2x80x94Fexe2x80x94Cr alloys and thereby effect significant improvements in intergranular corrosion and cracking resistance.
Studies have shown that certain xe2x80x9cspecialxe2x80x9d grain boundaries, described on the basis of the xe2x80x9cCoincident Site Latticexe2x80x9d model of interface structure (Kronberg and Wilson, Trans. Met. Soc. AIME, 185, 501 (1949)) as lying within xcex94xcex8 of xcexa3, where xcexa3xe2x89xa629 and xcex94xcex8xe2x89xa615xc2x0_xcexa3xe2x88x920.5 (Brandon, Acta Metall., 14, 1479, (1966)) are highly resistant to intergranular degradation processes such as corrosion, cracking, and grain boundary sliding; the latter being a principal contributor to creep deformation. The disclosure of Kronberg and Wilson and of Brandon are incorporated therein by reference to their teachings covering special grain boundaries.
We have discovered that finished and semi-finished articles made of austenitic Nixe2x80x94Fexe2x80x94Cr alloys, whether in the wrought, forged, cast or welded condition, may be subjected to working to induce deformation of the near surface region by a technique such as shot peening, followed by annealing of the article at a temperature below the melting point for a time sufficient to induce recrystallization in the cold-worked near surface region and increase the frequency of special low xcexa3 CSL grain boundaries.
In this specification, xe2x80x9cthe near surface regionxe2x80x9d refers to the surface layer of the article to a depth in the range of 0.01 mm to about 0.5 mm. xe2x80x9cWorkingxe2x80x9d will hereinafter be used in this specification as a shorthand reference to working to induce deformation.
It is a principal object of this invention to provide a surface treatment methodology which will alter the recrystallized structure in the near surface region of a finished article or component made austenitic Nixe2x80x94Fexe2x80x94Cr alloys to impact significant resistance to intergranular corrosion and cracking during the service of the article or component, without the need for bulk deformation thereof by a process of rolling, extruding, forging or the like. The hardness of the surface layer after the recrystallization treatment is lower than the hardness of the article before the processing.
It is a further object of this invention to provide a surface treatment process as aforesaid, which may be used to treat and improve the degradation and corrosion resistance of finished parts of complex shape and parts which may already be in service, in particular, nuclear steam generator tubes, nuclear reactor head penetrations and the like. Suitably treated parts also include weld clad components such as recovery boiler wall panels for the pulp and paper industry, and closure welds on canisters for nuclear waste storage.
The method of the present invention enhances the concentration of special grain boundaries in the surface of metallic articles. This is achieved without invoking conventional strengthening mechanisms, such as precipitation or age-hardening, and without substantially altering the tensile strength or hardness of the material. Typically the layer in which the special grain boundary fraction has been increased, exhibits a reduction in tensile strength, when compared to the as received material or the bulk of the material, which has not been subjected to this process.
Our experiments and reviews of the literature indicate that conventional surface cold working of articles of the kind with which we are herein concerned produces a special grain boundary fraction no greater than 10 to 15%. The method of the present invention allows this to be improved significantly more than 20%. Enhanced resistance to intergranular corrosion and cracking results when the special grain boundary fraction goes above 30% and typically 40% to 50%.
The treatment time required to achieve the desired properties varies, depending on the material, but typically ranges from 1 minute to 75 hours, and preferably from 5 minutes to 50 hours.
With a view of achieving these objects, there is provided a method for improving intergranular corrosion and cracking resistance of an article fabricated of an austenitic Nixe2x80x94Fexe2x80x94Cr alloy by subjecting the alloy to at least one cycle comprising the steps of:
(i) working the surface region of the article to a depth in the range of from 0.01 mm to about 0.5 mm; and
(ii) annealing the article at a temperature below the melting point of said alloy for a time sufficient to induce recrystallization in said surface region.